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Home Page > Clean Energy > Clean energy generation > Heat to electricity conversion


Heat to electricity conversion



As a gas expands through a turbine, it will rotate the turbine wheel and exit at a lower temperature and pressure. Much of the energy recovered using such turbo expanders is in fact the thermal energy drawn from the gas in the expansion process. Therefore, transferring available heat to a working fluid, which is then passed through a turbine to drive a generator, will produce electricity. Based on this, any high temperature source can be considered as potential electrical energy.


Imagine the huge energy potential lying unexploited beneath our feet: only 0.1% of our globe is colder than 100°C.

One km3 of 200°C hot granite cooled by 20°C delivers about 10 MW of electric power for a period of 20 years.

One km3 of 180°C hot geothermal water cooled to 100°C delivers about 7 MW of electric power for a period of 250 years.

Put another way, installed one MW is enough to supply 2500 homes every year.

Geothermal water has been used since the Roman time mainly for bathing, but nowadays it is also a means to produce electricity even for low temperature sources of about 100°C.

power electricity

Hot water coming from an aquifer source is pumped up. This natural source of heat is used to vaporise an appropriate working fluid mixture with a boiling point below 100°C. The vapour is then expanded through a radial inflow turbine, which drives a generator and delivers electricity to the grid. The low-pressure vapour is then condensed with an existing cold source (fresh water or ambient air) and the liquid is pumped back to the evaporator to begin the cycle again.

The hot geothermal water that was cooled by the cycle can be used for district heating and then returned to depth.

Cryostar’s high efficiency radial inflow turbine technology perfectly suits this application. Unlike axial turbines, the efficiency of Cryostar’s expanders can reach 90% or more.

Thanks to tools developed in-house , Cryostar engineers can optimise the closed loop binary cycle process (mainly working fluid and pressure level) and the expander to offer customers the optimum solution taking recovered power and cost into consideration.

Waste heat

A waste heat binary plant is quite similar to a geothermal binary plant, hence the thermal energy is coming from an unexploited heat source (100°C - 400°C typically), for example a thermal oil loop in an industrial plant, steel factory, cement factory, glass manufacturer, etc. Thanks to a closed loop binary cycle, the selected fluid mixture is expanded inside the Cryostar turbine to produce 1-15 MW of clean electricity.

To estimate potential electricity recovery, apply the following rule of thumb:

Potential sources:
Biomass boilers
Biogas engines
Residual heat from industry
Fuel exhaust heat

In a typical organic Rankine cycle (ORC) heat recovery system, the exhaust gas enters the evaporator where heat is transferred to the ORC working fluid in a shell and tube heat exchanger, or through an intermediate loop with thermal oil. The working fluid leaves the evaporator as superheated vapour and enters the expander, which starts to rotate. The expander’s shaft coupled to a generator produces electricity. When the fluid leaves the expander, a regenerator recovers some of its residual heat to preheat the incoming liquid. The ORC working fluid then enters the condenser. The resulting saturated liquid is pumped back and preheated by way of the regenerator before entering the evaporator once again.

Engineers use parabolic curved reflectors to concentrate solar rays on a receiver. With this technology, solar energy warms the transfer fluid flowing through the heat absorber tube to almost 400°C. This thermal energy need only be passed on to a working fluid from an ORC loop so that the turbo expander generator unit can use it to produce electricity.

Turbo generator single stage with lube oil bearings
- Power output range: 500 kW to 15 MW
- Speed range: 6 000 rpm to 33 000 rpm

Turbo generator double stages with lube oil bearings
- Power output range: 500 kW to 15 MW
- Speed range: 6 000 rpm to 33 000 rpm

Complete ORC or Kalina cycles
Not only can Cryostar provide its highly efficient turbo expander generators, it can also procure the whole closed loop binary system.


Germany 3.3 MW
France 1.5 MW

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